365 Days of Climate Awareness 53 - Introduction to Glaciers

A glacier is a mass of ice large enough to deform under its own weight. There is no set height when this happens, since the slope of the land matters. On a steep mountainside, 5 m can be sufficient. On flatter ground, 50 m is typical. Glaciers occur in a several settings: in mountains, where they are called alpine glaciers, on the plains, where they are called continental glaciers or, when over 50,000 km² in area, ice sheets, and floating on the ocean, where they are called ice shelves.

Glacier, aerial view.

The upper end of a glacier, where the flow of ice is initiated, is called the head. The lower terminus, which advances or retreats based on conditions, is the toe. The glacier itself is divided into two sections, the accumulation zone, where precipitation (snow) is greater than melting, leading to a net gain in ice mass, and the ablation zone, where melting and evaporation are greater than precipitation and result in a net loss.

Glacier, schematic.

Glaciers can move by two means: deformation, where the base is frozen to the ground below--a cold-based glacier--and layers above it slowly flow past. (This is closely analogous to the "boundary layer" theory of fluid motion, where liquid or gas immediately next to a stationary boundary is motionless due to friction, and the flow velocity increases steadily as you move away from that boundary, until you reach the maximal, "free stream" velocity. The zone where flow velocity is increasing is called the "boundary layer". Boundary layer theory is very important in the wind power industry.)

                             

Ohnoes, fluid dynamics! RUN! RUN AWAY NOW!

Glacial deformation can produce velocities as low as a few meters per year. The other means is by basal sliding, the dominant type of motion in warm-based glaciers, where water lubricates the ice-earth boundary and the glacier slides overhead. Basal sliding velocity can exceed 1.5 km/y. Factors such as accumulation rates, changing the stress throughout the glacier, or increased melting along the glacier's base, affect flow rate.

A glacier is an entire system of mass accumulation and wastage. Glaciers form by precipitation--rain and largely snow--continuing to fall in a constantly frozen region, and building up mass. The snow and ice droplets slowly accrete into a coarse, granular frozen material called firn, which is the midpoint between snow and solid ice. The firn, as it is buried increasingly deeply by more snow overhead, begins to deform into solid ice, and so enter the fabric of the glacier.

The ice begins to flow, away from the head and toward the toe. The rate of flow is extremely slow, and is measured in meters per year. But gradually, ice reaches the ablation zone where temperatures are warm enough--whether because of the air, or through contact with the water--that ice melts away. It either evaporates directly, or melts and flows out directly from the glacier or at its base, or through calving of icebergs into the ocean.

End moraine.

Glaciers have a number of effects on the underlying geology. The ice deforms around boulders or bedrock outcrops and "plucks" them, carrying sometimes cabin-sized rocks tens or hundreds of miles. Glaciers can also "polish" bedrock, as the sliding ice drags sediment and rock clasts across the bedrock, smoothing it over centuries. As glaciers slide over soil, they similarly pick up loads of sediment and transport that. The toe of an advancing glacier frequently bulldozes a large mound of sediment in front of it, which might be left behind as an end moraine, marking the extent of the glacier at one period of time. (A terminal moraine marks the furthest extent a glacier ever reached. There might be several end moraines behind it as the glacier continue to melt back.) A number of other sedimentary forms are also identified with glaciers, and they occur throughout the areas of North America which were once covered with ice.

Tomorrow: alpine glaciers.

Be well!